To increase the density of package structures, multiple dies may need to be packaged in a same package structure. To accommodate multiple dies, an interposer is typically used to bond dies thereon, and the resulting structure including the interposer and the dies are treated as a single structure.
It is desirable to form silicon interposers due to the fact that the technology for handling silicon substrates is mature. Further, features formed on silicon interposers may have a high density. However, due to various factors, such as the significant difference between the coefficient of thermal expansion (CTE) of silicon and the CTE of printed circuit board (PCB), it is difficult to bond a silicon interposer directly on a PCB without incurring reliability issues. For example, dies are bonded on a silicon interposer, which are bonded to PCB directly, the resulting package structure will suffer from reliability problems. A solution for the above-discussed problem is to insert an organic substrate between silicon interposer and PCB. This solution, however, still suffers from other issues. Firstly, organic substrates have a high cost. Secondly, organic substrates are typically soft, and hence may not be able to provide enough protection needed by other package components. Thirdly, the signal loss in organic substrates is high, particularly when the signal has a high frequency.